Photovoltaic devices, also known as solar cells, are used to harvest energy from sunlight. Silicon is by far the most popular material used. Photovoltaic devices can be classified into bulk as well as thin film types. Bulk silicon solar cells are made of single crystal silicon or multi-crystalline silicon wafers. Such wafers are of at least 100 micron thickness. As such they require considerable silicon material and are economically wasteful. Thin film solar cells are those fabricated with the silicon film grown on inexpensive substrates such as glass. They consume much less silicon as they are generally 5 microns or less in thickness. However their energy conversion efficiencies are lower than the bulk wafer type. Much improvement is needed in making thin film silicon solar cells.
The thin films can be amorphous (a-Si), multi-crystalline (MC), microcrystalline (mC), nanocrystalline (nC), or single crystalline (C) in nature. Each of these devices of course has its own merits and drawbacks. Amorphous silicon thin film solar cells, for example, are the most mature technology, but have relatively low energy conversion efficiency and suffer from long term degradation. Crystalline silicon solar cells require wafer transfer or true epitaxial growth or high temperature annealing, all of which are impractical for mass production purposes.
The present invention concerns multi-crystalline silicon thin film solar cells and photovoltaic devices. Multi-crystalline silicon thin films are used in many other electronic devices other than solar cells. For example, MC silicon is used in thin film transistors for driving liquid crystals and other active matrix displays. There is a significant body of literature on the fabrication of MC thin films on glass at low temperature while avoiding deformation of the glass substrate or superstrate.
The application of MC silicon to solar cells has been investigated extensively. Reviews of the development of MC silicon solar cells can be found in Aberle, “Fabrication and characterisation of crystalline silicon thin-film materials for solar cells,” Thin Solid Films, vol. 511-512, pp. 26-34, 2006, and Green et al, “Crystalline Silicon on Glass (CSG) Thin-Film Solar Cell Modules,” Solar Energy, vol. 77, pp. 857-863, 2004. Commercial thin film MC solar cells are available. Technologically, MC films can be obtained from amorphous silicon thin films by high temperature annealing and solid phase epitaxy (SPC), by excimer laser annealing (ELA), or by metal induced crystallization (MIC), which is essentially SPC with a catalyst to induce faster crystal growth at lower temperatures.
The main problem of MC material is the presence of grain boundaries which act as charge trapping sites. For a thick layer of MC film, there is always a random arrangement of grains. A current has to pass through the grain boundaries. Hydrogen passivation is conventionally employed to reduce charge trapping by the grain boundaries.
Another issue common to all thin film devices is the problem of surface reflection. Since the thin film is usually not much thicker than the absorption depth of the incoming photons, some or most of the light can penetrate without being absorbed. It is important to trap the light so that all or most photons can be eventually absorbed. In addition, due to the large refractive index mismatch between silicon and air, large surface reflection occurs, reducing the amount of light going into the silicon film. Texturing of the surface is usually needed to reduce surface reflection and to trap the light.